While transport networks traditionally rely on time-division multiplexing (TDM) technology, techniques have recently been developed which allow native transport of packet services such as Ethernet traffic. A technology, which allows Ethernet transport through a provider network is known as Provider Bridge Network (PBN), where Ethernet switching devices in a provider network are termed Provider Bridges.
Ethernet frames contain physical source and destination addresses termed MAC addresses, where MAC stands for Media Access Control, a protocol for controlling access at the data link layer 2 in the OSI model.
The basic Ethernet standard IEEE 802.1 defines MAC address learning, a service that characterizes a learning bridge, in which the source MAC address of a received packet and the interface at which that packet was received are stored so that future packets destined for that address can be forwarded only to the bridge interface on which that address is located. Packets destined for unrecognized addresses are forwarded out every other bridge interface different from the interface the packet has been received from. This scheme helps minimize traffic on the attached LANs.
Ethernet bridged networks may use VLAN tags added to the Ethernet frames to define different and isolated broadcast domains over the same physical infrastructure. This technique can be useful in transport network to isolate traffic from different customers. Since VLAN tags can already be used by customers, a technique called Q-in-Q defined in IEEE 802.1ad expands the VLAN space by tagging the tagged packets, thus producing a “double-tagged” frame. A first VLAN tag called C-VLAN is available for customer purposes and a second VLAN tag called S-VLAN is than added by the service provider. VLAN-based provider bridge networks suffer from the limitation of 4096 VLAN IDs, thus making them impractical for use in large backbone networks.
Another technique, which is called MAC-in-MAC and which is defined in IEEE 802.1ah, neatly avoids this Layer 2 scaling issue and eliminates the need for core and backbone switches to learn hundreds of thousands of MAC addresses. This is achieved by adding at the edge node of the provider network backbone provider MAC addresses to the Ethernet frame.
An alternative technology for native Layer 2 transport is known as virtual private LAN service (VPLS), which uses Multi Protocol Label Switching (MPLS) technology. The basic defining elements of VPLS are virtual switch instances (VSI) and pseudo wires (PW). The VSI is a virtual MAC bridging instance with “split horizon” learning and forwarding. Split horizon forwarding means unknown MAC addresses received from one PW will not be forwarded to any other PW. This avoids forwarding loops. The PW is used to transport traffic between VSIs in different nodes. A full mesh of PWs is required to ensure that all nodes can be reached.
A VPLS network is established by first defining the MPLS label switched path (LSP) tunnels which will support the VPLS PWs. This can be achieved through IP-based signaling protocols. Paths are first determined using the Open Shortest Path First (OSPF) link-state protocol, which selects the shortest path for the LSP to a target destination. A full bidirectional mesh of LSPs needs to be established between all participating VPLS provider edge (PE) nodes. Label Distribution Protocol (LDP) or Resource Reservation Protocol—Traffic Engineering (RSVP-TE) is then used to distribute the label information. At a next step, PWs are established over the existing LSPs. This can be achieved using LDP (IETF RFC4762) or Border Gateway Protocol BGP (IETF RFC4761) to exchange PW labels.
Transport networks typically have a control plane, which is based on the GMPLS protocol suite, an extension of the MPLS-TE control protocols for transport applications, including OSPF-TE and RSVP-TE. For example, OSPF-TE will take bandwidth availability into account when calculating the shortest path, while RSVP-TE allows reservation of bandwidth.
The VPLS architecture is generic and is applicable on any suitable packet switched network. It is therefore possible to envisage a VPLS-like approach based on other tunnel mechanisms, such as T-MPLS, MPLS-TP, and PBB-TE.
An extension of VPLS is termed hierarchical VPLS (H-VPLS), which was designed to address scalability issues in VPLS. In VPLS, all provider edge (PE) nodes are interconnected in a full mesh to ensure that all destinations can be reached. In H-VPLS, a new type of node is introduced called the multi-tenant unit (MTU), which aggregates multiple customer edge (CE) connections into a single PE.